Teenagers may be losing interest in Facebook, but Facebook doesn’t really need to worry. Instagram is a hit among tweens, and Facebook owns it.

In revealing financial results last week, Facebook admitted what many have sensed: Teens, especially younger teens, aren’t as interested in Facebook as they once were.

But is this a problem for Facebook? Maybe not. Instagram, a social network for photo sharing, is acting as a gateway to Facebook among kids who haven’t even started middle school.

Instagram’s popularity among the under-13 crowd is something of an open secret. Kids under 13 aren’t supposed to have Instagram accounts, according to the company’s terms of service, but many do. Even if these kids don’t have a smartphone, they can take and share photos with the iPod touch, a much-coveted device among that age group.

This under-13 age limit isn’t arbitrary. It is determined by the Children’s Online Privacy Protection Act, a federal law passed in 1998. The law is designed to protect kids’ privacy and keep them safe online.

It’s just entirely ineffective when all a kid needs to do is lie about her age when signing up.

Many months of undercover reporting — otherwise known as being a dad to a 10-year-old girl — have revealed an astonishing mix of ignorance and irresponsibility about Instagram among both tweens and their parents.

Parents sign up their kids for accounts without thinking through the implications of letting an 8-year-old post photos online without any supervision. Or kids, who are adept at using the internet on mobile devices, sign up for an account on their own without letting mom or dad know. (After all, their friends have accounts, so why shouldn’t they? Though a parent may have made clear that Facebook is off limits, the parent probably didn’t say anything about Instagram.) And many parents, let’s face it, find it a struggle to keep up with their kids’ use of the internet, especially given the number of apps available on portable devices.

Though Instagram didn’t even exist until about a dozen years after the Children’s Online Privacy Protection Act was passed, it’s just the type of online service that’s a problem for kids. They can post photos of themselves. And the photos, more likely than not, have location data embedded into them, meaning anyone can see precisely where the photo was snapped. Others kids can comment on these photos and, as happens, be savagely cruel about it.

Have you ever let a 9-year-old use your digital camera with her friends? I have. Trust me: They take photos of things you really don’t want to display to the world.

But point-and-shoot cameras aren’t really a problem, for the simple reason that they don’t funnel their photos straight to the internet.

Instagram does.

Don’t get me wrong: I’m a fan of Instagram. It’s just not for tweens.

Instagram addresses this problem with a webpage about underage children (at help.instagram.com/290666591035380/) explaining how to delete your child’s account. You can also report an underage user through an online form.

But maybe the company needs to go further, possibly by spreading the news to parents about Instagram’s age limit — and the sensible reason for this — on Facebook, through media interviews and elsewhere. Without a more pro-active stance by Instagram (and Facebook), you’ve got to wonder whether the company isn’t just courting tweens — who will, after all, soon be bonafide teenagers.

Another social photo service, Snapchat, altered its app to make it accessible to kids under 13. The app is famous for its self-destructing images; photos shared via Snapchat can only be viewed by the recipient for a few seconds. Now, if a kid under 13 tries to sign up, the child is given access to something called SnapKidz and allowed to add captions and drawings to images, but not send or receive photos.

Will any kid who wants to use Snapchat, a service known for being all about trading photos, fall for this? I doubt it.

At the same time, tweens keep flocking to social media websites, often without parental supervision or guidance. And they know how to hide what they’re doing. A study released earlier this year from online security firm McAfee found that 58 percent of kids from 10 to 12 think they know how to hide what they do online from their parents. About half have posted photos of themselves online.

The solution to this? It’s pretty simple, if you ask me, though many parents will disagree. Go to your kid’s mobile device. Search for Instagram. Delete the account. And set clear limits on what your kid can — and cannot — do on the internet, whether with a computer, a tablet or a smartphone.